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My Macbook (shared several users with separate logins) has a loose hinge which is evidenced by a few degrees of looseness when adjusting the cover

if I adjust position in the chair and the screen/cover moves the machine goes into

1) total freeze (6/10 times) (i.e. no keyboard control and cursor freezes in place, requiring 10 second hold power button shutdown (not possible to force quit anything, or use terminal to sudo shutdown -h now or any other technique)

or, if I'm luckier (2/10 times) it just result in
2) loss of internet connection, requiring restart to regain internet

the other 2/10 times nothing goes wrong.

any idea what the problem may be?...and more importantly what it might cost to fix, as I am on a tight budget and need to upgrade/replace computers for 3 people.

It's likely that the cabling running through the hinge assembly from the motherboard to the display has gotten crimped or damaged. If the signal from the display or wireless antenna gets corrupted from a physically mangled connection, it's not surprising the OS ends up shitting itself trying to recover.

If it's under warranty, take it to your closest Apple Store. If it's NOT under warranty and there's no liquid damage, repairing it will cost, at most, $300 (2010 pricing, don't quote me on that now).

Alternatively you can crack open the case and take a look at the cabling...it might just need a little adjusting.